In a direct injection system of the common-rail type, a high-pressure pump receives a flow of fuel from a tank by means of a low-pressure pump and feeds the fuel to a common rail hydraulically connected to a plurality of injectors. The pressure of the fuel inside the common rail must be constantly controlled according to the engine point either by varying the instantaneous flow rate of the high-pressure pump or by constantly feeding an excess of fuel to the common rail and by discharging the excess fuel from the common rail itself by means of a register. Generally, the solution of varying the instantaneous flow rate of the high-pressure pump is preferred, because it presents a much higher energy efficiency and does not cause an overheating of the fuel.
In order to vary the instantaneous flow rate of the high-pressure pump, there has been suggested a solution of the type presented in patent application EP0481964A1 or in U.S. Pat. No. 6,116,870A1 which describe the use of a variable flow rate high-pressure pump capable of feeding the common rail only with the amount of fuel needed to maintain the fuel pressure within the common rail equal to the desired value; specifically, the high-pressure pump is provided with an electromagnetic actuator capable of varying the flow rate of the high-pressure pump instant-by-instant by varying the closing moment of an intake valve of the high-pressure pump itself.
Alternatively, in order to vary the instantaneous flow rate of the high-pressure pump, it has been suggested to insert a flow rate adjusting device upstream of the pumping chamber comprising a continuously variable-section bottleneck, which bottleneck is controlled according to the required pressure within the common rail.
However, both the above-described solutions for varying the instantaneous flow rate of the high-pressure pump are mechanically complex and do not allow to adjust the instantaneous flow rate of the high-pressure pump with high accuracy. Furthermore, the flow rate adjustment device comprising a variable-section bottleneck presents a small introduction section in case of small flow rates and such a small introduction section determines a high local pressure loss (local load loss) which may compromise the correct operation of an intake valve which adjusts the fuel intake into a pumping chamber of the high-pressure pump.
For this reason, there has been suggested a solution of the type presented in patent application EP1612402A1, which relates to a high-pressure pump comprising a number of pumping elements operated in a reciprocating motion by means of corresponding intake and delivery strokes and in which each pumping element is provided with a corresponding intake valve in communication with an intake pipe fed by a low-pressure pump. On the intake pipe there is arranged a shut-off valve controlled in a choppered manner for adjusting the instantaneous flow rate of fuel fed to the high-pressure pump; in other words, the shut-off valve is a valve of the open/closed (on/off) type which is driven by modifying the ratio between the duration of the opening time and the duration of the closing time so as to vary the instantaneous flow rate of fuel fed to the high-pressure pump. In this manner, the shut-off valve always presents an effective and wide introduction section which does not determine an appreciable local pressure loss (local load loss).
In the various conditions of operation of the engine, the high-pressure pump needs to be able to precisely supply a very variable flow rate (no flow rate in “cut-off” operation or maximum flow rate in full-power operation); it is important for the fuel flow rate supplied by the high-pressure pump to be precise because the fuel flow rate supplied by the high-pressure pump directly effects the fuel pressure inside the common rail and thus any irregularity of the fuel flow rate supplied by the high-pressure pump determines a corresponding irregularity in the fuel pressure inside the common rail. In the direct injection systems of the common rail type currently marketed, provided with on/off type shut-off valve, it has been observed that the pressure of the fuel inside the common rail often presents irregularities at slow engine rates, i.e. when a small amount of fuel is injected by the injectors and thus the fuel flow rate supplied by the high-pressure pump is low.